Depolymerization of lignin – an enzymatic approach

Abstract:
In the last decades biomass has gained rising importance as renewable resource for fuels and platform chemicals. Whereas plant derived molecules like starch, sugars and proteins are well accessible for biotransformations the main components of wooden biomass - cellulose, hemicelluloses and lignin - are difficult to transform into valuable products. Lignin is one of the most abundant renewable raw materials available on earth and pulp and paper industry still produces millions of tons as a widely unutilised side product. Both, the high availability and the content of phenolic subunits, feature lignin as an attractive resource to obtain platform chemicals which nowadays are only supplied by petrochemical processes. The big challenge here is to break down the lignin polymer into its monolignolic subunits which than can be employed for further transformations. The following enzymatic approach to degrade lignin is pursued: Laccases are multinuclear copper-containing “blue” oxidases produced by certain white rot fungi. They catalyse a four electron transfer from a substrate over a mediator to oxygen which is reduced to water. Within this project a suitable reaction system for laccase-catalysed lignin degradation shall be validated. Therefore, different laccase-mediator-systems (LMS) are evaluated under varied conditions. To follow the degradation process several analytical methods are established: Cyclovoltammetry provides information about laccase-mediator-interactions. Particle size distribution analysis should visualize the degradation progress on suspended lignin particles in aqueous solution. Gel permeation chromatography shows lignin break down on molecular level. Detection of several degradation products is achieved by HPLC analysis. The results obtained by these methods should lead to first conclusions concerning the kinetic parameters of this process and in long term to a suitable reactor concept.

Keywords:
Lignin, Laccase, Depolymerization, Mediator